Overview
This lecture explains how to calculate the relative atomic mass of an element using the abundances and mass numbers of its isotopes.
Isotopes and Relative Atomic Mass
- Relative atomic mass is the average mass of an element's isotopes, weighted by their natural abundances.
- Carbon's relative atomic mass is close to 12 because 99% of its atoms are the carbon-12 isotope.
- Isotopic abundance refers to the percentage of an element's atoms that are a specific isotope.
Chlorine Isotope Example
- Chlorine exists as two isotopes: chlorine-35 and chlorine-37.
- Both isotopes have 17 protons; chlorine-35 has 18 neutrons, chlorine-37 has 20 neutrons.
- In a chlorine sample, 75% are chlorine-35 and 25% are chlorine-37.
Calculating Relative Atomic Mass
- Multiply each isotope's mass number by its percentage abundance.
- For chlorine: (35 × 75) + (37 × 25) = 2,625 + 925 = 3,550.
- Divide the total by 100 to account for the percentages: 3,550 ÷ 100 = 35.5.
- This gives chlorine a relative atomic mass of 35.5.
General Calculation Steps
- For any element: multiply each isotope’s mass number by its relative abundance.
- Sum these values for all isotopes.
- Divide the sum by 100 if abundances are given as percentages.
Key Terms & Definitions
- Isotope — Atoms of the same element with different numbers of neutrons.
- Relative atomic mass — The average mass of an element’s isotopes, weighted by their natural abundances.
- Isotopic abundance — The percentage of a particular isotope present in a natural sample.
Action Items / Next Steps
- Practice calculating relative atomic mass using data for other elements, like uranium.
- Write down all calculation steps when working on isotope problems.